With the explosive spread of broadband communication at homes, network contents are being increasingly diversified. This results in increased communication traffic and improved communication service, leading to daily growing needs for increased capacities and speeds and enhanced functions of communication networks, which support the increased traffic and the improved communication service. In recent years, optical communication techniques have played an important role for meeting these needs. Conventional optical networks are mostly terrestrial communication systems that connect two points together based on optical-to-electrical or electrical-to-optical signal processing. However, importantly, the current communication system needs to be further developed into a mesh type in the future; in the mesh communication system, on all the networks including access networks, multiple points are connected together with only optical signals without via any electric-to-optical signal conversion. Such a communication system makes a variety of communication utilization forms available for users.
Waveguide devices are components which have played important roles in the optical communication system. Application based on the principle of optical interference has allowed implementation of various functions such as an optical branch coupler, a wavelength multiplexer/demultiplexer, an interleave filter, an optical switch, and a variable optical attenuator (VOA). These devices are of a waveguide type and are thus flexible in circuit design and can be easily increased in scale and highly integrated. Furthermore, a process for manufacturing semiconductor components such as LSIs has been used for the devices. The devices are thus highly expected to be suitable for mass production. Among the various waveguides put into practical use, which are formed using semiconductors or polymer materials, those which are made of silica on a silicon substrate are characterized by low loss, high stability, and appropriate connections to optical fibers, and are thus most commercialized.
Reconfigurable add/drop multiplexing (ROADM) using wavelength division multiplexing (WDM) signals is a system for optical communication system nodes configured by using these waveguide devices. This system node has a function to deliver and receive any WDM channel signal to and from a lower layer network and then to transmit all signals to adjacent nodes. The ROADM system is used mainly to configure a ring network. Optical devices required to implement this function are a wavelength multiplexing/demultiplexing filter that multiplexes/demultiplexes WDM signals of different wavelengths, an optical switch configured to switch a signal path, a VOA configured to adjust the intensity of signal light, an optical transceiver/receiver, and a light intensity monitor. In particular, the wavelength multiplexing/demultiplexing filter, the optical switch, the VOA, and the like can be implemented by waveguide devices.
In recent years, these waveguide devices have been able to be integrated into one module to provide a sophisticated optical device that fulfills the main functions of an ROADM system. Every effort is being made to introduce such optical devices into actual network systems. FIG. 18 shows an example of such a device and is a block diagram of a circuit in which wavelength multiplexing/demultiplexing filters (1404, 1406, and 1416), optical switches (1408-1 to 1408-N), VOAs (1410-1 to 1401-N), optical couplers (1402 and 1412-1 to 1412-N), and monitor PDs (Photo Detectors) (1414-1 to 1414-N) are integrated into one module 1400. In the example illustrated in FIG. 18, a WDM signal entering a main path through an input (In) is first branched by the tapping optical coupler 1402. Subsequently, one of the branched signal is separated into signals of individual wavelengths by the drop wavelength demultiplexing (DEMUX) filter 1404, and only the signals of the wavelengths for use in the lower layer network are detected. The other signal is also separated into signals of the individual wavelengths by the DEMUX filter 1406. The resultant signals pass through the 2×1 optical switches 1408-1 to 1408-N each of which selects either the main path signal or an add path signal that is transmitted from the lower layer network. The 2×1 optical switches select the signal from the add path only for the wavelength corresponding to the wavelength signal detected on the drop path as described above. Moreover, the signals of the respective wavelengths have the signal power levels thereof adjusted by the VOAs 1410-1 to 1410-N. Output signals from the VOAs 1410-1 to 1410-N are partly monitored by the tapping optical couplers 1412-1 to 1412-N and the monitor PD 1414-1 to 1414-N connected to output sides of the respective VOAs 1410-1 to 1410-N. The monitored signals are fed back to control the attenuation of the VOAs. The signals of the respective wavelengths with the levels thereof adjusted are converted into a WDM signal by a wavelength multiplexing (MUX) filter 1416. The WDM signal then exits the device through an output (out) for the main path.
According to the conventional art, these individual optical devices are mounted in a module by being connected together via optical fibers. For further reduced device size and power consumption and further increased scale, a major challenge is to further increase the degree of integration.
One of the proposed techniques adapted to meet the need for the increased degree of integration is a multichip integration technique. The multichip integration technique directly connects individual substrates of waveguide device together without any optical fibers, thus reducing the size of the waveguide device itself and the footprint where the waveguide devices occupy in the module. For example, in the configuration in FIG. 18, the wavelength multiplexing/demultiplexing filters 1406 and 1416 are fabricated into one waveguide device substrate 1420. Similarly, the optical switches 1408-1 to 1408-N, the VOAs 1410-1 to 1410-N, and the optical couplers 1412-1 to 1412-N are fabricated into one waveguide device substrate 1430. Subsequently, the substrates 1420 and 1430 are connected together without any optical fibers. Furthermore, the monitor PDs 1414-1 to 1414-N are not waveguide devices but can be connected to monitor ports of the optical couplers 1412-1 to 1412-N on an end face of the wavelength multiplexing/demultiplexing filter substrate 1420 or on an end face of the substrate 1430 for the optical switches without via any optical fibers. The present technique enables a reduction in the length of optical fibers used in the module 1400 and in the number of elements used to connect the substrates 1420 and 1430 together. This leads to the reduced footprint in the module and the increased degree of integration of devices. In this case, the VOAs function to suppress a signal power level deviation among the channels by attenuating the passing signal lights to adjust their optical levels.
FIG. 19A shows the most basic configuration of a VOA as a waveguide device. The VOA 1500 is a Mach Zehnder interferometer (MZI)-type optical device including two directional couplers 1504 and 1508 that branch and combine optical signals and arm waveguides 1506a and 1506b with thin film heaters 1512a and 1512b formed thereon. An optical signal entering the VOA 1500 through a port 1502a is branched into two signals by the directional coupler 1504. The resultant signals propagate through the arm waveguides 1506a and 1506b, respectively, and are combined together again by the directional coupler 1508. At this time, when one of the thin film heaters 1512a and 1512b is supplied with electricity through an electrode pad 1516 or 1518, a phase difference occurs between the arm waveguides 1506a and 1506b. Then, based on a phase relationship in the directional coupler 1508, the intensity of an optical signal output to a port 1510a or 1510b changes. When the phase difference is 0, 100% of the optical signal exits to the port 1510b. When the phase difference is π, 100% of the optical signal exits to the port 1510a. The device can be functioned as a VOA by utilizing this phenomenon to adjust the phase difference by controlling the supply of electricity to the thin film heaters in an analog manner. FIG. 19B is a cross-sectional view taken along line XIXB-XIXB in FIG. 19A. An optical waveguide is fabricated on a silicon substrate 1520 and includes a cladding 1522 which is formed of silica glass and a rectangular core 1524 which is covered with the cladding 1522. Heat insulation grooves 1514 are formed on the respective sides of each arm waveguide by removing the cladding along the waveguide using an etching technique. The heat insulation grooves 1514 enable a reduction in power required for switching or attenuation. Here, according to the principle of MZI interference, to achieve a sufficient extinction ratio or optical attenuation even if an error occurs in a coupling rate as a result of a production error in the optical couplers, a path from the port 1502a to the port 1510b or a path from the port 1502b to the port 1510a (cross path) is commonly used as a main signal path. Moreover, with polarization dependence of power consumption or thermooptic effects being taken into account, it is most common to block optical signals or to make the device have the maximum attenuation when no electricity is conducted through the thin film heaters 1512a and 1512b. To achieve this, an appropriate difference in an optical distance over which optical signals propagate, that is, in an optical path length (optical path length difference) needs to be designed between the arm waveguides 1506a and 1506b. 
The optical attenuation operation in the VOA composed of an MZI as a basic element, including the two optical waveguides attenuates the optical level of the main port (an output waveguide connected to an optical fiber or another waveguide device) and allows unnecessary optical power (which results from the attenuation) to be output to the other port (dummy port). For example, in an MZI VOA using a cross path as a main signal path, if the port 1502a in FIG. 19A is used as an input, the port 1510b is used as a main port, and the port 1510a is used as a dummy port. According to the conventional techniques, in general, the unnecessary optical power guided to the dummy port propagates to the output end face of the waveguide device substrate, where the optical power is directly radiated to the air.